Aircraft are designed to provide crash protection for the occupants. This is normally accomplished by absorbing the impact energy imposed on the aircraft using airframe structures as well as crashworthy subsystems such as seats and landing gears. In addition to providing energy absorption, the airframe structures are designed to maintain their structural integrity, to provide a livable volume around the occupants during the crash, and to facilitate post-crash egress of the occupants from the aircraft. Military aircraft are also designed to withstand ballistic impacts without experiencing catastrophic structural failures. The airframe structure around internal fuel bladders of the aircraft as well as the belly structure are subjected to very high transverse pressure loads during the crash impacts. Ballistic impacts on the internal fuel bladders can also cause high transverse pressure loads on the surrounding airframe structure.
Many modern aircraft utilize composite structures to reduce the weight of the aircraft. However, typical composite structures utilize graphite face sheets with phenolic resin cores or solid laminates, which are brittle. Because they are brittle, these types of composite structures around internal fuel tanks rapidly fail under transverse pressure loads that occur during aircraft crashes and/or ballistic impacts on fuel tanks. Structural failures can have catastrophic effects on occupant survivability. These structural failures expose the occupants to potential injuries from major mass items such as engines, transmissions, and rotor systems due to a failed primary load path. Similar failures are also observed during ballistic tests of the fuel tanks and surrounding structures.
Another potential failure location of the composite structures is the aircraft belly skin panels. These skin failures occur particularly on crash impacts on soft soil and water. As the skins fail and rupture under transverse pressure loads, the impact loads are not transferred to the subfloor frame and keel beam structures that are part of the airframe energy absorption system. This results in higher airframe crash decelerations which can result in occupant spinal injuries. Furthermore, during crash impacts on water, rupture of the belly skin panels can lead to rapid sinking of the aircraft before the occupants can safely egress. Therefore, there is a need to improve the transverse pressure load capability of composite structures, such as the composite structures that might be used on aircraft.